Methods for Stabilizing Flow in Channels and System Thereof

ABSTRACT

A method and system for stabilizing flow includes introducing a flow into a channel with a minimum cross-sectional dimension of less than three millimeters and triggering a release of one or more bubbles in the flow at one or more locations in the channel. The one or more locations are spaced in from an inlet and an outlet to the channel.

This application is a divisional of U.S. patent application Ser. No.12/497,180, filed Jul. 2, 2009, which is a divisional of U.S. patentapplication Ser. No. 10/939,896, filed Sep. 13, 2004, now U.S. Pat. No.7,575,046, which claims the benefit of U.S. Provisional PatentApplication Ser. No. 60/504,267, filed Sep. 18, 2003, which are herebyincorporated by reference in their entirety.

FIELD OF THE INVENTION

The present invention generally relates to microchannels andminichannels and, more particularly, to methods and systems forstabilizing flow and/or improving heat transfer performance inmicrochannels and minichannels and systems thereof.

BACKGROUND

In a cooling system with a network of multiple parallel microchannelsand minichannels, each having a hydraulic diameter less than three mm, aliquid used for cooling is introduced. As the liquid flows through thenetwork, initially heat transfer is by convection from the walls of themicrochannels and minichannels.

As the liquid flows further downstream through the network, additionalheating of the liquid occurs. Eventually, the wall temperature of themicrochannels and minichannels rises above the local saturationtemperature of the liquid. However, boiling of the liquid does not occurunless there are proper nucleation cavities present. If one or morenucleation cavities are present, nucleation occurs over the nucleationcavity or cavities and the liquid boils. The range of possiblenucleation cavities in the microchannels and minichannels can beexpanded by the application of a sufficiently high degree of superheatto the microchannels and minichannels.

Prior to this nucleation occurring and during the superheating, theliquid in the microchannels and minichannels, at least in the vicinityof the nucleation sites, becomes superheated. At this point, a bubblepresent or formed in this liquid experiences a very rapid bubble growth.The rapid bubble growth leads to severe pressure fluctuation in themicrochannel or minichannel, which can result in a reverse flow of theliquid. Experimental evidence and a description of the mechanism leadingto this instability is described in Kandlikar, S. G. “Heat TransferMechanisms During Flow Boiling In Microchannels.” Proceedings of theFirst International Conference on Microchannels and Minichannels Apr.24-25, 2003, Rochester, N.Y., USA ICMM2003-1005, S. G. Kandlikar, EditorASME Publication, 2003, which is herein incorporated by reference in itsentirety. The rapid bubble growth may also adversely affect the heattransfer performance, including heat transfer degradation and/orreduction in critical heat flux, of the cooling system.

SUMMARY OF THE INVENTION

A method for stabilizing flow during flow boiling in accordance withembodiments of the present invention includes introducing a flow into achannel with a minimum cross-sectional dimension of less than threemillimeters and triggering a release of one or more bubbles in the flowat one or more locations in the channel to stabilize the flow. The oneor more locations are spaced in from an inlet and an outlet to thechannel.

A system for stabilizing flow during flow boiling in accordance withembodiments of the present invention includes the channel and thetriggering system. The channel has a minimum cross-sectional dimensionof less than three millimeters. The triggering system triggers a releaseof one or more bubbles in the flow at one or more locations in thechannel to stabilize the flow. The one or more locations are spaced infrom an inlet and an outlet to the channel.

The present invention provides a method and system for the efficientremoval of the heat potential of flow boiling in a channel or channels,such as microchannels and minichannels. The present invention overcomesthe severe oscillatory nature of the flow during flow boiling byinitiating the nucleation and flow boiling at specific locations in thechannel or channels. The locations are chosen such that the localsuperheat in the wall and/or surrounding liquid is relatively low anddoes not lead to the rapid bubble growth that leads to flow and pressureoscillations. Flow and pressure oscillations can lead to flow reversaland premature drying out and to a reduction in cooling performance. Toassist in initiating nucleation the present invention heats a regionwith or immediately preceding a location with nucleation cavities. Toprovide additional flow stability the present invention may alsoincorporate local pressure reduction devices.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a system with a low pressure zonefor stabilizing flow which is flowing from left to right in amicrochannel or minichannel in accordance with embodiments of thepresent invention;

FIGS. 2A and 2B are cross-sectional views of systems with a low pressurezone for stabilizing flow which is flowing from left to right in amicrochannel or minichannel in accordance with other embodiments of thepresent invention;

FIG. 3 is a cross-sectional view of the system shown in FIG. 1 with aheater and with a flow direction from left to right in accordance withother embodiments of the present invention;

FIGS. 4A and 4B are cross-sectional views of the systems with a lowpressure zone as shown in FIGS. 2A and 2B respectively with nucleationcavities and with a flow direction from left to right in accordance withother embodiments of the present invention;

FIG. 5 is a cross-sectional view of a system with nucleation cavitiesfor stabilizing flow in a microchannel or minichannel along one surfacein accordance with other embodiments of the present invention;

FIG. 6 is a cross-sectional view of a system with nucleation cavitiesfor stabilizing flow in a microchannel or minichannel in a systematic orrandom pattern in accordance with other embodiments of the presentinvention;

FIG. 7 is a cross-sectional view of a system with a low pressure zoneand nucleation cavities for stabilizing flow in a microchannel orminichannel in accordance with embodiments of the present invention; and

FIG. 8 is a cross-sectional view of a system for stabilizing flow in amicrochannel or minichannel in a systematic or random pattern with aheater in accordance with other embodiments of the present invention.

DETAILED DESCRIPTION

Systems 10(1)-10(10) for stabilizing flow F in one or more channels12(1)-12(10) in accordance with embodiments of the present invention areillustrated in FIGS. 1-8. The systems 10(1)-10(10) each have a channel12(1)-12(10) which each includes one or more low pressure devices14(1)-14(3), low pressure zones 16(1)-16(4), heating device 18(1)-18(2),and/or nucleation cavities 20(1)-20(7), although the systems10(1)-10(10) each can include other types and numbers of elementsarranged in other manners. The present invention provides a number ofadvantages including providing systems and methods for efficientlyremoving the heat potential of flow boiling in microchannels andminichannels. The present invention overcomes the severe oscillatorynature of the flow F during flow boiling by initiating the nucleationand flow boiling at specific locations in the channel or channels. Thelocations are chosen such that the local superheat in the wall of thechannel or channels and/or surrounding flow is relatively low and doesnot lead to the rapid growth of bubbles that lead to flow and pressureoscillations.

Referring more specifically to FIGS. 1-8, each of the channels12(1)-12(10) is either a minichannel or a microchannel. A minichannelhas a minimum cross-sectional dimension between about 200 microns tothree millimeters and a microchannel has a minimum dimension of about200 microns or less. The cross-sectional dimension is measured acrossthe channel in a direction which is substantially perpendicular to thedirection of the flow. Although hydraulic diameter is described in,Kandlikar, S. G. “Heat Transfer Mechanisms During Flow Boiling InMicrochannels.” Proceedings of the First International Conference onMicrochannels and Minichannels Apr. 24-25, 2003, Rochester, N.Y., USAICMM2003-1005, S. G. Kandlikar, Editor ASME Publication, 2003, the aboveclassification is used for microchannels and minichannels herein. Inthese embodiments, the channels 12(1)-12(10) each have a circular,cross-sectional shape, although each of the channels 12(1)-12(10) couldhave other cross-sectional shapes. The arrow F represents the flowflowing in the channels 12(1)-12(10) and also indicates the direction ofthat flow. A variety of different types of flow F, such as fluids, canpass through the channels 12(1)-12(10) and the flow F can go in otherdirections.

Referring to FIG. 1, the system 10(1) for stabilizing flow F includesthe channel 12(1) with the low pressure device 14(1), although system10(1) can include other types and numbers of elements arranged in othermanners. The flow F is a liquid in this and the other systems10(1)-10(10) described herein, although other types of mediums can beused for the flow F. The channel 12(1) has a wall which defines apassage 22(1) that is substantially straight and includes an inlet 24(1)and an outlet 26(1), although the channel 12(1) could have otherconfigurations, such as a curved shape, and other numbers of walls andopenings.

The low pressure device 14(1) is positioned in the channel 12(1) and isspaced in from the inlet 24(1) and the outlet 26(1), although othernumbers and types of pressure drop elements in other locations can beused. A pressure drop element, such as pressure device 14(1) refers toany element or configuration that creates a pressure drop, flashing,increased resistance to backflow, and/or creation of a low pressurezone. The flashing leads to the presence of vapor phase in the flowwhich prevents any further superheating of the wall and/or the flow F.Excess superheat is the cause for rapid bubble growth that leads toinstability in the flow F.

The low pressure device 14(1) extends fully or partially around theinner periphery of the channel 12(1) and forms a high pressure region28(1) upstream of the low pressure device 14(1) and forms a low pressureregion 30(1) downstream of the low pressure device 14(1). A passage32(1) extends through the low pressure device 14(1) to connect the highpressure region 28(1) to the low pressure region 30(1). The passage32(1) has a cone-shaped, inner periphery with the larger opening to thiscone-shaped, inner periphery facing the high pressure region 28(1),although the passage 32(1) could have other shapes and configurations.

Referring to FIG. 2A, the system 10(2) for stabilizing flow F includesthe channel 12(2) with the low pressure zone 16(1), although system10(2) can include other types and numbers of elements arranged in othermanners. Elements in FIG. 2A which correspond to those described withreference to FIG. 1 will have like reference numerals. The channel 12(2)has a wall which defines a passage 22(2) that has a high pressure region28(2) which is narrower than and upstream from a low pressure region30(2). The passage 22(2) also includes an inlet 24(2) and an outlet26(2), although the channel 12(2) and passage 22(2) could have otherconfigurations with other numbers, shapes, and types of regions andother numbers of walls and openings. The low pressure zone 16(1) isadjacent the transition from the high pressure region 28(2) to the lowpressure region 30(2). A passage 32(2) connects the high pressure region28(2) to the low pressure region 30(2).

Referring to FIG. 2B, the system 10(3) for stabilizing flow F includesthe channel 12(3) with the low pressure zone 16(2), although system10(3) can include other types and numbers of elements arranged in othermanners. Elements in FIG. 2B which correspond to those described withreference to FIGS. 1 and 2A will have like reference numerals. Thechannel 12(3) has a wall which defines a passage 22(3) that has a highpressure region 28(3) which is wider than and upstream from a lowpressure region 30(3). The passage 22(3) also includes an inlet 24(3)and an outlet 26(3), although the channel 12(3) and passage 22(3) couldhave other configurations with other numbers, shapes, and types ofregions and other numbers of walls and openings. The low pressure zone16(2) is adjacent the transition from the high pressure region 28(3) tothe low pressure region 30(3). A passage 32(3) connects the highpressure region 28(2) to the low pressure region 30(2).

Referring to FIG. 3, the system 10(4) for stabilizing flow F includesthe channel 12(4) with the low pressure device 14(2), the heating device18(1) and the nucleation cavities 20(1), although system 10(4) caninclude other types and numbers of elements arranged in other manners.Elements in FIG. 3 which correspond to those described with reference toFIGS. 1-2B will have like reference numerals. The channel 12(4) has awall which defines a passage 22(4) that is substantially straight andincludes an inlet 24(4) and an outlet 26(4), although the channel 12(4)could have other configurations, such as a curved shape, and othernumbers of walls and openings.

The low pressure device 14(2) is positioned in the channel 12(4) and isspaced in from the inlet 24(4) and the outlet 26(4), although othernumbers and types of pressure drop elements in other locations can beused as described earlier. The low pressure device 14(2) extends fullyor partially around the inner periphery of the channel 12(4) and forms ahigh pressure region 28(4) upstream of the low pressure device 14(2) andforms a low pressure region 30(4) downstream of the low pressure device14(2). A passage 32(4) extends through the low pressure device 14(2) toconnect the high pressure region 28(4) to the low pressure region 30(4).The passage 32(4) has a cone-shaped, inner periphery with the largeropening to this cone-shaped, inner periphery facing the high pressureregion 28(4), although the passage 32(4) could have other shapes andconfigurations.

The heating device 18(1) is positioned around the wall of the channel22(4) adjacent the low pressure device 14(2) and is used to superheatthe adjacent portion of the channel, although other numbers and types ofheating systems in other locations could be used. The heating device18(1) is also positioned over nucleation cavities 20(1) which arelocated in the wall of the channel 22(4), although other numbers andlocations for the nucleation cavities and other types of reentrant ornucleation sites can be used. The actual size and shape of thenucleation cavities 20(1) is based on the geometry of the channel 22(4)and the range of flow conditions that the channel 22(4) is subject to.

Referring to FIG. 4A, the system 10(5) for stabilizing flow F includesthe channel 12(5) with the low pressure zone 16(3), although system10(5) can include other types and numbers of elements arranged in othermanners. Elements in FIG. 4A which correspond to those described withreference to FIGS. 1-3 will have like reference numerals. The channel12(5) has a wall which defines a passage 22(5) that has a high pressureregion 28(5) which is narrower than and upstream from a low pressureregion 30(5). The passage 22(5) also includes an inlet 24(5) and anoutlet 26(5), although the channel 12(5) and passage 22(5) could haveother configurations with other numbers, shapes, and types of regionsand other numbers of walls and openings. The low pressure zone 16(3) isadjacent the transition from the high pressure region 28(5) to the lowpressure region 30(5). A passage 32(5) connects the high pressure region28(5) to the low pressure region 30(5).

Nucleation cavities 20(2) are located in the wall of the channel 22(5)adjacent the low pressure zone 16(3), although other numbers andlocations for the nucleation cavities and other types of reentrant ornucleation sites can be used. The actual size and shape of thenucleation cavities 20(2) is based on the geometry of the channel 22(5)and the range of flow conditions that the channel 22(5) is subject to.

Referring to FIG. 4B, the system 10(6) for stabilizing flow F includesthe channel 12(6) with the low pressure zone 16(4), although system10(6) can include other types and numbers of elements arranged in othermanners. Elements in FIG. 4B which correspond to those described withreference to FIGS. 1-4A will have like reference numerals. The channel12(6) has a wall which defines a passage 22(6) that has a high pressureregion 28(6) which is wider than and upstream from a low pressure region30(6). The passage 22(6) also includes an inlet 24(6) and an outlet26(6), although the channel 12(6) and passage 22(6) could have otherconfigurations with other numbers, shapes, and types of regions andother numbers of walls and openings. The low pressure zone 16(4) isadjacent the transition from the high pressure region 28(6) to the lowpressure region 30(6). A passage 32(6) connects the high pressure region28(6) to the low pressure region 30(6).

Nucleation cavities 20(3) are located in the wall of the channel 22(6)adjacent the low pressure zone 16(4), although other numbers andlocations for the nucleation cavities and other types of reentrant ornucleation sites can be used. The actual size and shape of thenucleation cavities 20(3) is based on the geometry of the channel 22(6)and the range of flow conditions that the channel 22(6) is subject to.

Referring to FIG. 5, the system 10(7) for stabilizing flow F includesthe channel 12(7) with nucleation cavities 20(4), although system 10(7)can include other types and numbers of elements arranged in othermanners. Elements in FIG. 5 which correspond to those described withreference to FIGS. 1-4B will have like reference numerals. The channel12(7) has a wall which defines a passage 22(7) that is substantiallystraight and includes an inlet 24(7) and an outlet 26(7), although thechannel 12(7) could have other configurations, such as a curved shape,and other numbers of walls and openings.

Nucleation cavities 20(4) are spaced apart substantially the samedistance along a section of the wall of the channel 22(7), althoughother numbers and locations for the nucleation cavities and other typesof reentrant or nucleation sites can be used. The actual size and shapeof the nucleation cavities 20(4) is based on the geometry of the channel22(7) and the range of flow conditions that the channel 22(7) is subjectto.

Referring to FIG. 6, the system 10(8) for stabilizing flow F includesthe channel 12(8) with nucleation cavities 20(5), although system 10(8)can include other types and numbers of elements arranged in othermanners. Elements in FIG. 6 which correspond to those described withreference to FIGS. 1-5 will have like reference numerals. The channel12(8) has a wall which defines a passage 22(8) that is substantiallystraight and includes an inlet 24(8) and an outlet 26(8), although thechannel 12(8) could have other configurations, such as a curved shape,and other numbers of walls and openings.

Nucleation cavities 20(5) are randomly located along a section of thewall of the channel 22(8), although other numbers and locations for thenucleation cavities and other types of reentrant or nucleation sites canbe used. The actual size and shape of the nucleation cavities 20(5) isbased on the geometry of the channel 22(8) and the range of flowconditions that the channel 22(8) is subject to.

Referring to FIG. 7, the system 10(9) for stabilizing flow F includesthe channel 12(9) with the low pressure device 14(3) and nucleationcavities 20(6), although system 10(9) can include other types andnumbers of elements arranged in other manners. Elements in FIG. 7 whichcorrespond to those described with reference to FIGS. 1-6 will have likereference numerals. The channel 12(9) has a wall which defines a passage22(9) that is substantially straight and includes an inlet 24(9) and anoutlet 26(9), although the channel 12(9) could have otherconfigurations, such as a curved shape, and other numbers of walls andopenings.

The low pressure device 14(3) is positioned in the channel 12(9) and isspaced in from the inlet 24(9) and the outlet 26(9), although othernumbers and types of pressure drop elements in other locations can beused as described earlier. The low pressure device 14(3) extends fullyor partially around the inner periphery of the channel 12(9) and forms ahigh pressure region 28(7) upstream of the low pressure device 14(3) andforms a low pressure region 30(7) downstream of the low pressure device14(3). A passage 32(7) extends through the low pressure device 14(3) toconnect the high pressure region 28(7) to the low pressure region 30(7).The passage 32(7) has a cone-shaped, inner periphery with the largeropening to this cone-shaped, inner periphery facing the high pressureregion 28(7), although the passage 32(7) could have other shapes andconfigurations.

Nucleation cavities 20(6) are spaced apart substantially the samedistance along a section of the wall of the channel 22(9), althoughother numbers and locations for the nucleation cavities and other typesof reentrant or nucleation sites can be used. The actual size and shapeof the nucleation cavities 20(6) is based on the geometry of the channel22(9) and the range of flow conditions that the channel 22(9) is subjectto.

Referring to FIG. 8, the system 10(10) for stabilizing flow F includesthe channel 12(10) the heating device 18(2) and the nucleation cavities20(7), although system 10(10) can include other types and numbers ofelements arranged in other manners. Elements in FIG. 8 which correspondto those described with reference to FIGS. 1-7 will have like referencenumerals. The channel 12(10) has a wall which defines a passage 22(10)that is substantially straight and includes an inlet 24(10) and anoutlet 26(10), although the channel 12(10) could have otherconfigurations, such as a curved shape, and other numbers of walls andopenings.

The heating device 18(2) is positioned around the wall of the channel12(10) adjacent the nucleation cavities 20(7), although other numbersand types of heating systems in other locations could be used.

Nucleation cavities 20(7) are randomly located along a section of thewall of the channel 22(10), although other numbers and locations for thenucleation cavities and other types of reentrant or nucleation sites canbe used. The actual size and shape of the nucleation cavities 20(7) isbased on the geometry of the channel 22(10) and the range of flowconditions that the channel 22(10) is subject to.

With the systems 10(1)-10(10) described above, the instability in theflow F is reduced and performance improvement is achieved by triggeringan earlier nucleation in the flow F. The triggered early nucleation inthe systems 10(1)-10(10) results in smaller vapor bubbles or slugs thatare separated by relatively uniform liquid slugs and that do not growtoo rapidly. The smaller vapor bubbles or slugs improve the heattransfer performance in the systems 10(1)-10(10) because the liquid filmof the small vapor bubbles or slugs covering the wall or walls in thechannels 12(1)-12(10) does not completely evaporate and is able totransfer heat before leaving the region. As a result, degradation in thecooling performance of systems 10(1)-10(10) is avoided.

The rapid growth of bubbles leads to reversed flow of vapor into aninlet manifold coupled to one or more of the channels 12(1)-12(10). Thisleads to flow instabilities and flow maldistribution in parallelchannels.

The process of nucleation depends on the availability of nucleationcavities of the right size and shape which satisfy the nucleationcriteria as described in an equation proposed by Hsu and Graham,rewritten in the following form by Kandlikar (Handbook of Phase Change,Taylor and Francis, 1999, which is herein incorporated by reference inits entirety) has the following form, and it provides the cavity radiirange that can nucleate under a given set of local conditions. Thisequation, referred to as Equation 1 or eq. 1 herein, is as follows:

$r_{\max}^{*},{r_{\min}^{*} = {\frac{1}{2}\lbrack {\frac{\Delta \; T_{sat}^{*}}{{\Delta \; T_{sat}^{*}} + {\Delta \; T_{sub}^{*}}} \pm \sqrt{( \frac{\Delta \; T_{sat}^{*}}{{\Delta \; T_{sat}^{*}} + {\Delta \; T_{sub}^{*}}} )^{2} - \frac{1}{( {{\Delta \; T_{sat}^{*}} + {\Delta \; T_{sub}^{*}}} )}}} \rbrack}}$

where

r*=r/δ _(t)

ΔT* _(sat) =ΔT _(sat) h _(lv)δ_(t)/(8σT _(sat) v _(lv))

ΔT* _(sub) =ΔT _(sub) h _(lv)δ_(t)/(8σT _(sat) v _(lv))

r—cavity mouth radius,

δ_(t)—thickness of the thermal boundary layer, approximately=h/k, whereh is the single phase heat transfer coefficient prior to nucleation andk is the thermal conductivity of liquid

ΔT_(sat)—wall superheat, degree C.

h_(lv)—latent heat, J/kg

σ—surface tension, N/m

T_(sat)—saturation temperature, K

v_(lv)—change in specific volume during evaporation, m³/kg

ΔT_(sub)—local liquid subcooling, degree C.

r_(max) and r_(min) are the non-dimensional minimum and maximum cavitymouth radii that will nucleate according to criteria described in eq.(1). A number of modifications to the above criteria are available, suchas having the temperature at the tip of the bubble protruding in theflow F to be at least equal to or higher than the saturationtemperature. The nucleation criterion is also modified for a channel orchannels that are not uniform over the circumference, such as a channelor channels with rectangular cross-section, and for a channel orchannels where the local wall and flow temperature fields vary withcircumferential location.

The operation of the system 10(1) for stabilizing flow F will bedescribed with reference to FIG. 1. The location where the wall and/orthe flow F is expected to be slightly superheated (within a fewdegrees), such that flashing occurs, may be identified. The low pressuredevice 14(1) can be positioned in the channel 12(1) before that locationand spaced in from the inlet 24(1) and outlet 26(1) to the channel12(1).

Next, the flow F enters the inlet 24(1) to the channel 12(1) and flowsfrom the high pressure region 28(1) to the low pressure region 30(1)through the passage 32(1) in the low pressure device 14(1). The flow Fheading towards the low pressure device 14(1) is kept in single phaseflow by insulating the inner surface of the channel 12(1) so thatnucleation or two-phase flow does not occur prior to passing through thelow pressure device 14(1). The heat gain in the high pressure region28(1) of the channel 12(1) is also controlled to keep the flow F fromboiling.

The low pressure zone upstream from and adjacent to the low pressuredevice 14(1) triggers flashing to occur which leads to the presence ofvapor phase, i.e. bubbles, in the flow F. This prevents any furthersuperheating of the wall of the channel 12(1) and/or flow F. The flow Fwith the bubbles is able to effectively transfer heat to the flow Fthrough the wall of the channel 12(1) resulting in improved heattransfer characteristics when compared with prior systems. The lowpressure device 14(1) also increases the resistance to backflow in thechannel 12(1) to provide further flow stability.

In the system 10(1), a release of bubbles can also optionally beobtained by vibrating the flow in at least a portion of the channel12(1). A variety of different types of systems and device could be usedto vibrate the flow F in the channel, such as a vibrating devicedisposed in a portion of the flow F in the channel 12(1) or the walls ofthe channel 12(1).

The operation of the system 10(2) for stabilizing flow F will bedescribed with reference to FIG. 2A. The operation of the system 10(2)is the same as the system 10(1), except as described herein. The flow Fenters the inlet 24(2) to the channel 12(2) and flows from the highpressure region 28(2) to the low pressure region 30(2) through thepassage 32(2). The low pressure zone upstream from and adjacent to thetransition from the high pressure region 28(2) to the low pressureregion 30(2) triggers flashing to occur which leads to the presence ofvapor phase, i.e. bubbles, in the flow F. This prevents any furthersuperheating of the wall of the channel 12(2) and/or flow F. The flow Fwith the bubbles is able to effectively transfer heat to the flow Fthrough the wall of the channel 12(2) resulting in improved heattransfer characteristics when compared with prior systems. Thisconfiguration of the channel 12(2) with the high pressure region 28(2)of the channel being narrower than the low pressure region 30(2) alsoincreases the resistance to backflow in the channel 12(2) to providefurther flow stability.

The operation of the system 10(3) for stabilizing flow F will bedescribed with reference to FIG. 2B. The operation of the system 10(3)is the same as the system 10(2), except as described herein. The flow Fenters the inlet 24(3) to the channel 12(3) and flows from the highpressure region 28(3) to the low pressure region 30(3) through thepassage 32(3). The low pressure zone upstream from and adjacent to thetransition from the high pressure region 28(3) to the low pressureregion 30(3) triggers flashing to occur which leads to the presence ofvapor phase, i.e. bubbles, in the flow F. This prevents any furthersuperheating of the wall of the channel 12(3) and/or flow F. The flow Fwith the bubbles is able to effectively transfer heat to the flow Fthrough the wall of the channel 12(3) resulting in improved heattransfer characteristics when compared with prior systems. Thisconfiguration of the channel 12(3) with the high pressure region 28(3)of the channel being narrower than the low pressure region 30(3) alsoincreases the resistance to backflow in the channel 12(3) to providefurther flow stability.

The operation of the system 10(4) for stabilizing flow F will bedescribed with reference to FIG. 3. The operation of the system 10(4) isthe same as the system 10(1), except as described herein. Again, thelocation where the wall and/or the flow F is expected to be slightlysuperheated (within a few degrees), such that flashing occurs, may beidentified. The low pressure device 14(2) can be positioned in thechannel 12(4) before that location and spaced in from the inlet 24(4)and outlet 26(4) to the channel 12(4).

Nucleation cavities 20(1) are formed in the wall of the channel 12(4) ata location where the superheat of the flow F is moderate to initiatenucleation over the nucleation cavities 20(1), but is not large enoughto create modest or severe instability in the flow F due to latenucleation. The mouth opening to at least some of nucleation cavities20(1) fall within those prescribed by eq. (1) described earlier herein.A larger range of diameters for nucleation cavities 20(1) may be placedindividually or in clusters at the desired locations to allow for slightdepartures from eq. (1) due to variations in fluid properties and toallow for uncertainties and other assumptions made (including uniformheat transfer coefficient over the perimeter) in deriving eq. (1) and toallow for a range of operating conditions, including flow rates, heatfluxes, operating pressure, and inlet conditions. The nucleationcavities 20(1) can be fabricated using a variety of differenttechniques, such as laser drilling, etching, deep ion etching, laserablation, sintering, scraping and fin bending, roughness, orindentation. The heating device 18(1) is positioned around the channel12(4) adjacent the location of the nucleation cavities 20(1). Thenucleation cavities 20(1) can also have different sizes and shapes toinitiate nucleation under different conditions and at differentlocations.

Once the system 10(4) is formed, the flow F enters the inlet 24(4) tothe channel 12(4) and flows from the high pressure region 28(4) to thelow pressure region 30(4) through the passage 32(4) in the low pressuredevice 14(2). The heating device 18(1) heats the wall of the channeladjacent the location of the nucleation cavities 20(1). Heating thenucleation cavities 20(1) helps to initiate nucleation in the flow F.The heating device 18(1) could be supplied with essentially constantpower or with power pulses to release bubbles over the nucleationcavities 20(1) periodically to initiate boiling and reduce the level ofsuperheat attained by the flow F. The period of bubble release isdetermined so that the pressure oscillations in the flow F are reducedto prevent flow reversal or other detrimental effects of large superheatbuildup prior to nucleation. Although a heating device 18(1) is shown,other mechanisms for bubble release can be used, such as mechanismswhich use vibrations, laser light, and/or ultrasound.

The low pressure zone upstream from and adjacent to the low pressuredevice 14(2) triggers flashing to occur which leads to the presence ofvapor phase, i.e. bubbles, in the flow F. This nucleation and flashingprevents any further superheating of the wall of the channel 12(4)and/or flow F. The flow F with the bubbles is able to effectivelytransfer heat to the flow F through the wall of the channel 12(4)resulting in improved heat transfer characteristics when compared withprior systems. The low pressure device 14(2) also increases theresistance to backflow in the channel 12(4) to provide further flowstability.

The operation of the system 10(5) for stabilizing flow F will bedescribed with reference to FIG. 4A. The operation of the system 10(5)is the same as the system 10(2), except as described herein. Asdescribed in greater detail with reference to the nucleation cavities20(1) in system 10(4), nucleation cavities 20(2) are formed in the wallof the channel 12(5) at a location where the superheat of the flow F ismoderate to initiate nucleation over the nucleation cavities 20(2), butis not large enough to create modest or severe instability in the flow Fdue to late nucleation.

The flow F enters the inlet 24(5) to the channel 12(5) and flows fromthe high pressure region 28(5) to the low pressure region 30(5) throughthe passage 32(5). The low pressure zone upstream from and adjacent tothe transition from the high pressure region 28(5) to the low pressureregion 30(5) along with the nucleation at the nucleation cavities 20(2)triggers flashing to occur which leads to the presence of vapor phase,i.e. bubbles, in the flow F. This prevents any further superheating ofthe wall of the channel 12(5) and/or flow F. The flow F with the bubblesis able to effectively transfer heat to the flow F through the wall ofthe channel 12(5) resulting in improved heat transfer characteristicswhen compared with prior systems. This configuration of the channel12(5) with the high pressure region 28(5) of the channel being narrowerthan the low pressure region 30(5) also increases the resistance tobackflow in the channel 12(5) to provide further flow stability.

The operation of the system 10(6) for stabilizing flow F will bedescribed with reference to FIG. 4B. The operation of the system 10(6)is the same as the system 10(3), except as described herein. Asdescribed in greater detail with reference to the nucleation cavities20(1) in system 10(4), nucleation cavities 20(3) are formed in the wallof the channel 12(6) at a location where the superheat of the flow F ismoderate to initiate nucleation over the nucleation cavities 20(3), butis not large enough to create modest or severe instability in the flow Fdue to late nucleation

The flow F enters the inlet 24(6) to the channel 12(6) and flows fromthe high pressure region 28(6) to the low pressure region 30(6) throughthe passage 32(6). The low pressure zone upstream from and adjacent tothe transition from the high pressure region 28(6) to the low pressureregion 30(6) along with the nucleation at the nucleation cavities 20(3)triggers flashing to occur which leads to the presence of vapor phase,i.e. bubbles, in the flow F. This prevents any further superheating ofthe wall of the channel 12(6) and/or flow F. The flow F with the bubblesis able to effectively transfer heat to the flow F through the wall ofthe channel 12(6) resulting in improved heat transfer characteristicswhen compared with prior systems. This configuration of the channel12(6) with the high pressure region 28(6) of the channel being narrowerthan the low pressure region 30(6) also increases the resistance tobackflow in the channel 12(6) to provide further flow stability.

The operation of the system 10(7) for stabilizing flow F will bedescribed with reference to FIG. 5. The operation of the system 10(7) isthe same as the system 10(4), except as described herein. As describedin greater detail with reference to the nucleation cavities 20(1) insystem 10(4), nucleation cavities 20(4) are formed in a substantiallyuniform pattern along a section of the wall of the channel 12(7) wherethe superheat of the flow F is moderate to initiate nucleation over thenucleation cavities 20(4), but is not large enough to create modest orsevere instability in the flow F due to late nucleation.

Once the system 10(7) is formed, the flow F enters the inlet 24(7) andflows through the channel 12(7). The nucleation cavities 20(4) initiatenucleation in the flow F. This nucleation prevents any furthersuperheating of the wall of the channel 12(7) and/or flow F. The flow Fwith the bubbles is able to effectively transfer heat to the flow Fthrough the wall of the channel 12(7) resulting in improved heattransfer characteristics when compared with prior systems.

The operation of the system 10(8) for stabilizing flow F will bedescribed with reference to FIG. 6. The operation of the system 10(8) isthe same as the system 10(7), except as described herein. As describedin greater detail with reference to the nucleation cavities 20(1) insystem 10(4), nucleation cavities 20(5) are formed in a random patternalong a section of the wall of the channel 12(8) where the superheat ofthe flow F is moderate to initiate nucleation over the nucleationcavities 20(5), but is not large enough to create modest or severeinstability in the flow F due to late nucleation.

Once the system 10(8) is formed, the flow F enters the inlet 24(8) andflows through the channel 12(8). The nucleation cavities 20(5) initiatenucleation in the flow F. This nucleation prevents any furthersuperheating of the wall of the channel 12(8) and/or flow F. The flow Fwith the bubbles is able to effectively transfer heat to the flow Fthrough the wall of the channel 12(8) resulting in improved heattransfer characteristics when compared with prior systems.

The operation of the system 10(9) for stabilizing flow F will bedescribed with reference to FIG. 7. The operation of the system 10(9) isthe same as the system 10(1), except as described herein. The locationwhere the wall and/or the flow F is expected to be slightly superheated(within a few degrees), such that flashing occurs, may be identified.The low pressure device 14(3) can be positioned in the channel 12(9)before that location and spaced in from the inlet 24(9) and outlet 26(9)to the channel 12(9).

As described in greater detail with reference to the nucleation cavities20(1) in system 10(4), nucleation cavities 20(6) are formed in asubstantially uniform pattern along a section of the wall of the channel12(9) where the superheat of the flow F is moderate to initiatenucleation over the nucleation cavities 20(6), but is not large enoughto create modest or severe instability in the flow F due to latenucleation.

Next, the flow F enters the inlet 24(9) to the channel 12(9) and flowsfrom the high pressure region 28(7) to the low pressure region 30(7)through the passage 32(7) in the low pressure device 14(3). The lowpressure zone upstream from and adjacent to the low pressure device14(3) along with the nucleation at the nucleation cavities 20(6)triggers flashing to occur which leads to the presence of vapor phase,i.e. bubbles, in the flow F. This prevents any further superheating ofthe wall of the channel 12(9) and/or flow F. The flow F with the bubblesis able to effectively transfer heat to the flow F through the wall ofthe channel 12(9) resulting in improved heat transfer characteristicswhen compared with prior systems. The low pressure device 14(1) alsoincreases the resistance to backflow in the channel 12(9) to providefurther flow stability.

The operation of the system 10(10) for stabilizing flow F will bedescribed with reference to FIG. 8. The operation of the system 10(10)is the same as the system 10(4), except as described herein. Asdescribed in greater detail with reference to the nucleation cavities20(1) in system 10(4), nucleation cavities 20(7) are formed in a randompattern along a section of the wall of the channel 12(10) where thesuperheat of the flow F is moderate to initiate nucleation over thenucleation cavities 20(7), but is not large enough to create modest orsevere instability in the flow F due to late nucleation. The heatingdevice 18(2) is positioned around the channel 12(10) adjacent thelocation of the nucleation cavities 20(7).

Once the system 10(10) is formed, the flow F enters the inlet 24(10) andflows through the channel 12(10). The heating device 18(2) heats thewall of the channel adjacent the location of the nucleation cavities20(7). Heating the nucleation cavities 20(7) helps to initiatenucleation in the flow F. This nucleation prevents any furthersuperheating of the wall of the channel 12(10) and/or flow F. The flow Fwith the bubbles is able to effectively transfer heat to the flow Fthrough the wall of the channel 12(10) resulting in improved heattransfer characteristics when compared with prior systems.

Another way for improving heat transfer performance and stability insystems, such as system 10(1)-10(10) involves the use of dissolved gasesin the flow F. The dissolved gases helps in early nucleation and therebylimit the superheat of the flow F and bubble growth rate after bubbleformation. The flow F containing dissolved gases can be used eitheralone with naturally occurring nucleation cavities or can be used inconjunction with other embodiments described herein. The dissolved gasesform bubbles that attach on the wall of the channel and/or are in theflow F thus effectively creating interfaces between liquid and gas orgas vapor mixture where evaporation can occur at relatively low liquidand/or wall superheats.

Yet another way for improving heat transfer performance and stabilityinvolves the introduction of microbubbles in the flow F. Themicrobubbles may be made of gases that are not soluble, or have limitedsolubility in the liquid. Any technique for generation of microbubblescan be implemented. The presence of microbubbles limits the liquidsuperheat as the liquid evaporates at the bubble interface and limitsthis liquid superheat. The bubbles may attach on the wall and/or flow inthe liquid thus effectively creating interfaces between liquid and gasor gas vapor mixture where evaporation can occur at relatively lowliquid and/or wall superheats.

The present invention provides methods and systems to stabilize the flowduring flow boiling in a channel or channels. The systems 10(1)-10(10)described herein are merely exemplary and other combinations of theteachings in each can be used. The present invention utilizes pressurereduction and/or strategically placed nucleation cavities to achieveflow boiling under stable and workable operating conditions. The presentinvention can be used during flow boiling in any channel or channels toachieve stable flow and efficient heat removal. The various methods andsystems for stabilizing flow, such as the methods and systems which uselow pressure zone(s), use one or more nucleation cavities, heat portionsor all of the channel(s), introduce non-soluble gases, microbubbles, orhigher volatile liquid, can each be combined with one or more of theother embodiments to provide further flow stability.

Having thus described the basic concept of the invention, it will berather apparent to those skilled in the art that the foregoing detaileddisclosure is intended to be presented by way of example only, and isnot limiting. Various alterations, improvements, and modifications willoccur and are intended to those skilled in the art, though not expresslystated herein. These alterations, improvements, and modifications areintended to be suggested hereby, and are within the spirit and scope ofthe invention. Additionally, the recited order of processing elements orsequences, or the use of numbers, letters, or other designationstherefor, is not intended to limit the claimed processes to any orderexcept as may be specified in the claims. Accordingly, the invention islimited only by the following claims and equivalents thereto.

What is claimed is:
 1. A system for stabilizing flow during flowboiling, the system comprising: at least one of a minichannel and amicrochannel having channel walls between an inlet and an outletdefining a passage capable of receiving flow; a high pressure regionupstream from a transition to a low pressure region within the passage;a low pressure zone within the passage adjacent the transition from thehigh pressure region to the low pressure region; one or more nucleationcavities having a radius within a range which satisfies criteria fornucleation located in the wall of the channel adjacent the low pressurezone fashioned to trigger a release of one or more bubbles in the flowat one or more locations in the at least one of the minichannel and themicrochannel that effectively transfer heat to the flow through the wallof the channel and increase resistance to backflow in the channel andstabilize the flow.
 2. The system as set forth in claim 1, furthercomprising a vibrating system adjacent the low pressure zone.
 3. Thesystem as set forth in claim 1, further comprising a heating deviceadjacent the low pressure zone.
 4. The system as set forth in claim 2,further comprising a heating device adjacent the low pressure zone. 5.The system as set forth in claim 1, wherein one or more of the criteriafor nucleation are based on at least one of a geometry of the at leastone of the minichannel and the microchannel and a range of conditionsfor the flow.
 6. The system as set forth in claim 1, further comprisingat least one insulator upstream from the low pressure zone on at least aportion of an inner surface of the at least one of the minichannel andthe microchannel.
 7. The system as set forth in claim 1, wherein the atleast one of the minichannel and the microchannel is a minichannel witha minimum cross-sectional dimension of less than three millimeters. 8.The system as set forth in claim 1, wherein the at least one of theminichannel and the microchannel is a microchannel with a minimumcross-sectional dimension of less than about 200 microns.
 9. The systemas set forth in claim 1, further comprising additional nucleationcavities spaced apart substantially the same distance along a section ofthe channel wall.
 10. The system as set forth in claim 1, furthercomprising additional nucleation cavities randomly located along asection of the channel wall.
 11. The system as set forth in claim 1,wherein the flow further comprises dissolved gasses.
 12. The system asset forth in claim 1, wherein the flow further comprises microbubbles.13. The system as set forth in claim 1, wherein the flow furthercomprises non-soluble gasses.
 14. The system as set forth in claim 1,wherein the flow further comprises volatile liquid.
 15. The system asset forth in claim 1, wherein the low pressure zone is downstream fromthe transition from the high pressure region to the low pressure region.16. The system as set forth in claim 1, wherein the nucleation cavitiesare in the low pressure region downstream from the low pressure zone.